1. Field of the Invention
The invention relates to a driving force transmission control system that controls and transmits driving force that is input into an input shaft and transmitted to an output shaft such that the amount of driving force to be transmitted is variable.
2. Description of the Related Art
There is a conventional driving force transmission control system that includes a conversion mechanism which converts rotative force generated by an electric motor into axial thrust force, and that transmits driving force generated by an engine from an input shaft to an output shaft such that the amount of driving force to be transmitted is variable, by pressing a friction clutch with the thrust force. A driving force transmission control system of this type is mounted in, for example, a four-wheel-drive vehicle provided with main drive wheels to which the driving force generated by the engine is constantly transmitted and auxiliary drive wheels to which the driving force generated by the engine is transmitted depending on a vehicle travelling state, and is used to adjust the driving force that is transmitted to the auxiliary drive wheels. A driving force transmission control system of this type is described in, for example, Published Japanese Translation of PCT application No. 2005-527741.
In the driving force transmission control system described in Published Japanese Translation of PCT application No. 2005-527741, the rotational position of an electric motor is computed from the torque that should be transmitted by a friction clutch on the basis of a characteristic curve indicating a prescribed relationship between transmission torque transmitted by the friction clutch in which clutch plates are lubricated with a lubricant, and the rotational position of the electric motor. Then, the torque to be transmitted by the friction clutch is accurately and promptly adjusted by outputting a current value corresponding to the rotational position to the electric motor to control driving of the electric motor.
However, it was found by the inventors of the invention that, in the driving force transmission control system having the above-described configuration, even when the current supplied to the electric motor is constant, the torque to be transmitted by the friction clutch varies depending on the temperature. One of the factors of the variations is the influence of the variations in the viscosity of a lubricant present between the clutch plates due to the temperature variations.
Generally, the viscosity of the lubricant used to lubricate the clutch plates varies depending on the temperature. The viscosity of the lubricant is low when the temperature is high, whereas the viscosity of the lubricant is high when the temperature is low. Thus, the torque that is transmitted by the friction clutch is influenced by the variations in the viscosity of the lubricant due to the temperature variations. A relatively high torque is transmitted to the output shaft when the temperature of the lubricant is low, for example, when the engine is started in a cold region. This causes a possibility that the driving force will not be appropriately distributed to front wheels and rear wheels and fuel efficiency will be lowered with an increase in rotational resistance in a differential mechanism located on the auxiliary drive wheel side.